Bacteria assemble remarkable surface structures that interface with their surrounding environment. One such structure is the glycolipid known as lipopolysaccharide (LPS) that covers the surface of gram-negative bacteria. LPS is anchored to the bacterial cell by its lipid anchor known as lipid A. Lipid A is synthesized via a highly conserved pathway at the cytoplasmic face of the inner membrane. This is followed by addition of the core oligosaccharide and transport of the molecule across the inner membrane. The O-antigen polysaccharide is ligated to the core-lipid A in the periplasm completing LPS assembly. During the trafficking of LPS to the bacterial surface, latent enzymes modify the LPS structure contributing towards the diversity seen in LPS structure. For the most part, these enzymes target the lipid A anchor and the inner core oligosaccharide domains of the molecule. Since the lipid A is the bioactive component of LPS, these modifications can have a profound impact on disease, by altering LPS recognition by the mammalian innate immune receptor TLR4- MD2. Additionally, alteration of the LPS structure can directly impact the outer membrane permeability barrier, and bacterial resistance to host antimicrobial peptides. The overall objective of this proposal is to unravel the molecular mechanisms by which two pathogenic organisms, Helicobacter pylori and Campylobacter jejuni, modify their LPS structure and the role these modifications play in virulence. Although related, these pathogens have evolved unique modification machinery perhaps adapted for their specific ecological niche. The specific aims of the current proposal are: (1) characterization of Helicobacter pylori LPS modification machinery; (2) characterization of lipid A modifications in Campylobacter jejuni; and (3) impact of Helicobacter and Camyplobacter LPS remodeling on the host innate immune response. The completion of the Aims below will directly contribute to our understanding of how LPS modification machinery impacts pathogenesis. Finally, from this work will come new avenues of vaccine development and the ability to generate engineered LPS structures that could serve as potential adjuvants and/or LPS antagonists.